Transversely-excited film bulk acoustic resonator using yx-cut lithium niobate for high power applications

ABSTRACT

Acoustic resonator devices, filters, and methods are disclosed. An acoustic resonator includes a substrate and a lithium niobate (LN) plate having front and back surfaces and a thickness ts. The back surface faces the substrate. A portion of the LN plate forms a diaphragm spanning a cavity in the substrate. An interdigital transducer (IDT) is on the front surface of the LN plate with interleaved fingers of the IDT on the diaphragm. The LN plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic wave in the diaphragm. Euler angles of the LN plate are [0°, β, 0°], where 0≤β≤60°. A thickness of the interleaved fingers of the IDT is greater than or equal to 0.8 ts and less than or equal to 2.0 ts.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 17/022,042, filedSep. 15, 2020, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR USING YX-CUT LITHIUM NIOBATE FOR HIGH POWER APPLICATIONS,which claims priority from provisional patent application 63/026,824,filed May 19, 2020, entitled IDT SIDEWALL ANGLE TO CONTROL SPURIOUSMODES IN XBARS.

Application Ser. No. 17/022,042 is a continuation-in-part of applicationSer. No. 16/829,617, entitled HIGH POWER TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATORS ON Z-CUT LITHIUM NIOBATE, filed Mar. 25, 2020, nowU.S. Pat. No. 10,868,512, which is a continuation of application Ser.No. 16/578,811, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATORS FOR HIGH POWER APPLICATIONS, filed Sep. 23, 2019, now U.S.Pat. No. 10,637,438.

Application Ser. No. 17/022,042 is also a continuation in part ofapplication Ser. No. 16/782,971, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR USING ROTATED Y-X CUT LITHIUM NIOBATE, filed Feb. 5,2020, now U.S. Pat. No. 10,790,802, which claims priority from62/904,133, filed Sep. 23, 2019, entitled WIDE BAND BAW RESONATORS ON120-130 Y-X LITHIUM NIOBATE SUBSTRATES.

Application Ser. No. 16/782,971 is a continuation-in-part of applicationSer. No. 16/689,707, entitled BANDPASS FILTER WITH FREQUENCY SEPARATIONBETWEEN SHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYER THICKNESS,filed Nov. 20, 2019, now U.S. Pat. No. 10,917,070, and is a continuationin part of application Ser. No. 16/438,141, filed Jun. 11, 2019,entitled SOLIDLY-MOUNTED TRANSVERSELY-EXCITED FILM BULK ACOUSTICRESONATOR, now U.S. Pat. No. 10,601,392, which claims priority fromapplication 62/818,564, filed Mar. 14, 2019, entitled SOLIDLY MOUNTEDXBAR and application 62/753,809, filed Oct. 31, 2018, entitled SOLIDLYMOUNTED SHEAR-MODE FILM BULK ACOUSTIC RESONATOR.

Application Ser. No. 16/438,141 is also a continuation-in-part ofapplication Ser. No. 16/230,443.

Application Ser. No. 16/578,811 is a continuation-in-part of applicationSer. No. 16/230,443, and application Ser. No. 16/689,707 is acontinuation of application Ser. No. 16/230,443.

Application Ser. No. 16/230,443, entitled TRANSVERSELY-EXCITED FILM BULKACOUSTIC RESONATOR, filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192,claims priority from the following provisional patent applications:application 62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR(XBAR); application 62/701,363, filed Jul. 20, 2018, entitled SHEAR-MODEFBAR (XBAR); application 62/741,702, filed Oct. 5, 2018, entitled 5 GHZLATERALLY-EXCITED BULK WAVE RESONATOR (XBAR); application 62/748,883,filed Oct. 22, 2018, entitled SHEAR-MODE FILM BULK ACOUSTIC RESONATOR;and application 62/753,815, filed Oct. 31, 2018, entitled LITHIUMTANTALATE SHEAR-MODE FILM BULK ACOUSTIC RESONATOR. All of theseapplications are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ (Long Term Evolution) specification defines frequency bands from3.3 GHz to 5.9 GHz. These bands are not presently used. Future proposalsfor wireless communications include millimeter wave communication bandswith frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic plan view and two schematic cross-sectional views ofa transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3 is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 4 is a graphic illustrating a shear primary acoustic mode in anXBAR.

FIG. 5 is a schematic block diagram of a filter using XBARs.

FIG. 6 is a schematic cross-sectional view of two XBARs illustrating afrequency-setting dielectric layer.

FIG. 7 is a chart of the e14 and e15 piezoelectric coefficients of alithium niobate plate with Euler angles [0°, β, 0° ] as functions of β.

FIG. 8 is a chart comparing the admittances of XBARs formed on rotatedY-X lithium niobate and Z cut lithium niobate.

FIG. 9 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs using rotated Y-X cut lithium niobatewithout a front-side dielectric layer.

FIG. 10 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs using rotated Y-X cut lithium niobatewith a front side dielectric layer with a thickness of 0.2 times athickness of the piezoelectric diaphragm.

FIG. 11 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs using rotated Y-X cut lithium niobatewith a front side dielectric layer with a thickness of 0.3 times athickness of the piezoelectric diaphragm.

FIG. 12 is a graph identifying preferred combinations of aluminum IDTthickness and IDT pitch for XBARs using rotated Y-X cut lithium niobatewith a front side dielectric layer with a thickness of 0.35 times athickness of the piezoelectric diaphragm.

FIG. 13 is a flow chart of a process for fabricating an acousticresonator or filter using rotated Y-X cut lithium niobate.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are well suited for use infilters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having a front surface 112 and aback surface 114. The front and back surfaces are essentially parallel.“Essentially parallel” means parallel to the extent possible withinnormal manufacturing tolerances. The piezoelectric plate 110 is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate 110 is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates 110 are Z-cut, whichis to say the Z axis is normal to the front surface 112 and back surface114. However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations including rotated Z-cut and rotatedYX-cut.

The back surface 114 of the piezoelectric plate 110 is attached to asurface 122 of the substrate 120 except for a portion of thepiezoelectric plate 110 that forms a diaphragm 115 spanning a cavity 140formed in the substrate 120. The portion of the piezoelectric plate 110that spans the cavity is referred to herein as the “diaphragm” due toits physical resemblance to the diaphragm of a microphone. As shown inFIG. 1, the diaphragm 115 is contiguous with the rest of thepiezoelectric plate 110 around all of a perimeter 145 of the cavity 140.In this context, “contiguous” means “continuously connected without anyintervening item”.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be attached to the surface 122 of thesubstrate 120 using a wafer bonding process. Alternatively, thepiezoelectric plate 110 may be grown on the substrate 120 or otherwiseattached to the substrate. The piezoelectric plate 110 may be attacheddirectly to the substrate or may be attached to the substrate 120 viaone or more intermediate material layers.

The cavity 140 is an empty space within a solid body of the XBAR 100.The cavity 140 may be a hole completely through the substrate 120 (asshown in Section A-A and Section B-B) or a recess in the substrate 120(as shown subsequently in FIG. 3). The cavity 140 may be formed, forexample, by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. An IDT is an electrode structure for convertingbetween electrical and acoustic energy in piezoelectric devices. The IDT130 includes a first plurality of parallel elongated conductors,commonly called “fingers”, such as finger 136, extending from a firstbusbar 132. The IDT 130 includes a second plurality of fingers extendingfrom a second busbar 134. The first and second pluralities of parallelfingers are interleaved. The interleaved fingers overlap for a distanceAP, commonly referred to as the “aperture” of the IDT. Thecenter-to-center distance L between the outermost fingers of the IDT 130is the “length” of the IDT.

The term “busbar” refers to the conductors that interconnect the firstand second sets of fingers in an IDT. As shown in FIG. 1, each busbar132, 134 is an elongated rectangular conductor with a long axisorthogonal to the interleaved fingers and having a length approximatelyequal to the length L of the IDT. The busbars of an IDT need not berectangular or orthogonal to the interleaved fingers and may havelengths longer than the length of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 ofthe piezoelectric plate that spans, or is suspended over, the cavity140. As shown in FIG. 1, the cavity 140 has a rectangular shape with anextent greater than the aperture AP and length L of the IDT 130. Acavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may more or fewer than foursides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. An XBAR for a 5G device willhave more than ten parallel fingers in the IDT 110. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 110.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated in the drawings.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may be formed on the front side of thepiezoelectric plate 110. The “front side” of the XBAR is the surfacefacing away from the substrate. The front-side dielectric layer 214 hasa thickness tfd. The front-side dielectric layer 214 is formed betweenthe IDT fingers 238. Although not shown in FIG. 2, the front sidedielectric layer 214 may also be deposited over the IDT fingers 238. Aback-side dielectric layer 216 may be formed on the back side of thepiezoelectric plate 110. The back-side dielectric layer 216 has athickness tbd. The front-side and back-side dielectric layers 214, 216may be a non-piezoelectric dielectric material, such as silicon dioxideor silicon nitride. tfd and tbd may be, for example, 0 to 500 nm. tfdand tbd are typically less than the thickness ts of the piezoelectricplate. tfd and tbd are not necessarily equal, and the front-side andback-side dielectric layers 214, 216 are not necessarily the samematerial. Either or both of the front-side and back-side dielectriclayers 214, 216 may be formed of multiple layers of two or morematerials.

The IDT fingers 238 may be one or more layers of aluminum, asubstantially aluminum alloy, copper, a substantially copper alloy,beryllium, gold, molybdenum, or some other conductive material. Thin(relative to the total thickness of the conductors) layers of othermetals, such as chromium or titanium, may be formed under and/or overthe fingers to improve adhesion between the fingers and thepiezoelectric plate 110 and/or to passivate or encapsulate the fingers.The busbars (132, 134 in FIG. 1) of the IDT may be made of the same ordifferent materials as the fingers. As shown in FIG. 2, the IDT fingers238 have rectangular cross-sections. The IDT fingers may have some othercross-sectional shape, such as trapezoidal.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e., the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness ts of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3 is an alternative cross-sectional view along the section planeA-A defined in FIG. 1. In FIG. 3, a piezoelectric plate 310 is attachedto a substrate 320. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the substrate. The cavity 340does not fully penetrate the substrate 320. Fingers of an IDT aredisposed on the diaphragm 315. The cavity 340 may be formed, forexample, by etching the substrate 320 before attaching the piezoelectricplate 310. Alternatively, the cavity 340 may be formed by etching thesubstrate 320 with a selective etchant that reaches the substratethrough one or more openings (not shown) provided in the piezoelectricplate 310. In this case, the diaphragm 315 may be contiguous with therest of the piezoelectric plate 310 around a large portion of aperimeter 345 of the cavity 340. For example, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around at least50% of the perimeter 345 of the cavity 340.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. A radio frequency (RF) voltage is applied to the interleavedfingers 430. This voltage creates a time-varying electric field betweenthe fingers. The direction of the electric field is primarily lateral,or parallel to the surface of the piezoelectric plate 410, as indicatedby the arrows labeled “electric field”. Since the dielectric constant ofthe piezoelectric plate is significantly higher than the surroundingair, the electric field is highly concentrated in the plate relative tothe air. The lateral electric field introduces shear deformation, andthus strongly excites a shear-mode acoustic mode, in the piezoelectricplate 410. Shear deformation is deformation in which parallel planes ina material remain parallel and maintain a constant distance whiletranslating relative to each other. A “shear acoustic mode” is anacoustic vibration mode in a medium that results in shear deformation ofthe medium. The shear deformations in the XBAR 400 are represented bythe curves 460, with the adjacent small arrows providing a schematicindication of the direction and magnitude of atomic motion. The degreeof atomic motion, as well as the thickness of the piezoelectric plate410, have been greatly exaggerated for ease of visualization. While theatomic motions are predominantly lateral (i.e. horizontal as shown inFIG. 4), the direction of acoustic energy flow of the excited primaryshear acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. High piezoelectric coupling enables the design andimplementation of microwave and millimeter-wave filters with appreciablebandwidth.

FIG. 5 is a schematic circuit diagram and layout for a high frequencyband-pass filter 500 using XBARs. The filter 500 has a conventionalladder filter architecture including three series resonators 510A, 510B,510C and two shunt resonators 520A, 520B. The three series resonators510A, 510B, and 510C are connected in series between a first port and asecond port (hence the term “series resonator”). In FIG. 5, the firstand second ports are labeled “In” and “Out”, respectively. However, thefilter 500 is bidirectional and either port may serve as the input oroutput of the filter. The two shunt resonators 520A, 520B are connectedfrom nodes between the series resonators to ground. A filter may containadditional reactive components, such as inductors, not shown in FIG. 5.All the shunt resonators and series resonators are XBARs. The inclusionof three series and two shunt resonators is exemplary. A filter may havemore or fewer than five total resonators, more or fewer than threeseries resonators, and more or fewer than two shunt resonators.Typically, all of the series resonators are connected in series betweenan input and an output of the filter. All of the shunt resonators aretypically connected between ground and the input, the output, or a nodebetween two series resonators.

In the exemplary filter 500, the three series resonators 510A, B, C andthe two shunt resonators 520A, B of the filter 500 are formed on asingle plate 530 of piezoelectric material bonded to a silicon substrate(not visible). Each resonator includes a respective IDT (not shown),with at least the fingers of the IDT disposed over a cavity in thesubstrate. In this and similar contexts, the term “respective” means“relating things each to each”, which is to say with a one-to-onecorrespondence. In FIG. 5, the cavities are illustrated schematically asthe dashed rectangles (such as the rectangle 535). In this example, eachIDT is disposed over a respective cavity. In other filters, the IDTs oftwo or more resonators may be disposed over a single cavity.

Each of the resonators 510A, 510B, 510C, 520A, 520B in the filter 500has resonance where the admittance of the resonator is very high and ananti-resonance where the admittance of the resonator is very low. Theresonance and anti-resonance occur at a resonance frequency and ananti-resonance frequency, respectively, which may be the same ordifferent for the various resonators in the filter 500. Inover-simplified terms, each resonator can be considered a short-circuitat its resonance frequency and an open circuit at its anti-resonancefrequency. The input-output transfer function will be near zero at theresonance frequencies of the shunt resonators and at the anti-resonancefrequencies of the series resonators. In a typical filter, the resonancefrequencies of the shunt resonators are positioned below the lower edgeof the filter's passband and the anti-resonance frequencies of theseries resonators are position above the upper edge of the passband.

FIG. 6 is a schematic cross-sectional view through a shunt resonator anda series resonator of a filter 600 that uses a dielectric frequencysetting layer to separate the resonance frequencies of shunt and seriesresonators. A piezoelectric plate 610 is attached to a substrate 620.Portions of the piezoelectric plate 610 form diaphragms spanningcavities 640 in the substrate 620. Interleaved IDT fingers, such asfinger 630, are formed on the diaphragms. A first dielectric layer 650,having a thickness t1, is formed over the IDT of the shunt resonator.The first dielectric layer 650 is considered a “frequency settinglayer”, which is a layer of dielectric material applied to a firstsubset of the resonators in a filter to offset the resonance frequenciesof the first subset of resonators with respect to the resonancefrequencies of resonators that do not receive the dielectric frequencysetting layer. The dielectric frequency setting layer is commonly SiO₂but may be silicon nitride, aluminum oxide, or some other dielectricmaterial. The dielectric frequency setting layer may be a laminate orcomposite of two or more dielectric materials.

A second dielectric layer 655, having a thickness t2, may be depositedover both the shunt and series resonator. The second dielectric layer655 serves to seal and passivate the surface of the filter 600. Thesecond dielectric layer may be the same material as the first dielectriclayer or a different material. The second dielectric layer may be alaminate or composite of two or more different dielectric materials.Further, as will be described subsequently, the thickness of the seconddielectric layer may be locally adjusted to fine-tune the frequency ofthe filter 600. Thus, the second dielectric layer can be referred to asthe “passivation and tuning layer”.

The resonance frequency of an XBAR is roughly proportional to theinverse of the total thickness of the diaphragm including thepiezoelectric plate 610 and the dielectric layers 650, 655. Thediaphragm of the shunt resonator is thicker than the diaphragm of theseries resonator by the thickness t1 of the dielectric frequency settinglayer 650. Thus, the series resonator will have a lower resonancefrequency than the shunt resonator. The difference in resonancefrequency between series and shunt resonators is determined by thethickness t1.

This patent is directed to XBAR devices on lithium niobate plates havingEuler angles [0°, β, 0° ]. For historical reasons, this plateconfiguration is commonly referred to as “Y-cut”, where the “cut angle”is the angle between the y axis and the normal to the plate. The “cutangle” is equal to β+90°. For example, a plate with Euler angles [0°,30°, 0° ] is commonly referred to as “120° rotated Y-cut”.

FIG. 7 is a graph 700 of two piezoelectric stress coefficients e15 ande16 for lithium niobate plates having Euler angles [0°, β, 0° ]. Thesolid line 710 is a plot of piezoelectric stress coefficient e15relating electric field along the x axis to shear stress or torque aboutthe y axis as a function of β. This shear stress excites the shearprimary acoustic mode shown in FIG. 4. The dashed line 720 is a plot ofpiezoelectric stress coefficient e16 relating electric field along the xaxis to shear stress or torque about the z axis as a function of β. Thisshear stress excites horizontal shear modes (e.g. the SH0 plate mode)with atomic displacements normal to the plane of FIG. 4, which areundesired parasitic modes in an XBAR. Note that these two curves areidentical and shifted by 90°.

Inspection of FIG. 7 shows that the first piezoelectric stresscoefficient is highest for Euler angle β about 30°. The firstpiezoelectric stress coefficient is higher than about 3.8 (the highestpiezoelectric stress coefficient for an unrotated Z-cut lithium niobate)for 0°≤β≤60°. The second piezoelectric stress coefficient is zero forEuler angle β about 30°, where the first piezoelectric stresscoefficient is maximum. In this context “about 30°” means “within areasonable manufacturing tolerance of 30°”. The second piezoelectricstress coefficient is less than about 10% of the first piezoelectricstress coefficient for 26°≤β≤34°.

FIG. 8 is a chart 800 showing the normalized magnitude of the admittance(on a logarithmic scale) as a function of frequency for two XBAR devicessimulated using finite element method (FEM) simulation techniques. Thedashed line 820 is a plot of the admittance of an XBAR on a Z-cutlithium niobate plate. In this case, the Z crystalline axis isorthogonal to the surfaces of the plate, the electric field is appliedalong the Y crystalline axis, and the Euler angles of the piezoelectricplate are 0, 0, 90°. The solid line 810 is a plot of the admittance ofan XBAR on a 120° Y-cut lithium niobate plate. In this case, theelectric field is applied along the X crystalline axis, which lies inthe plane of the surfaces of the lithium niobate plate. The YZ plane isnormal to the surfaces of the plate. The Z crystalline axis is inclinedby 30° with respect to orthogonal to the surfaces of the plate, and theEuler angles of the piezoelectric plate are 0°, 30°, 0°. In both cases,the plate thickness is 400 nm, and the IDT fingers are aluminum 100 nmthick. The substrate supporting the piezoelectric plate is silicon witha cavity formed under the IDT fingers.

The difference between anti-resonance and resonance frequencies of theresonator on the rotated Y-cut plate (solid line 810) is about 200 MHzgreater than the difference between anti-resonance and resonancefrequencies of the resonator on the Z-cut plate (dashed line 820). Theelectromechanical coupling of the XBAR on the rotated Y-cut plate isabout 0.32; the electromechanical coupling of the XBAR on the Z-cutplate is about 0.24.

U.S. Pat. No. 10,637,438 describes XBAR resonators for use in high powerapplications. U.S. Pat. No. 10,637,438 also describes the use of afigure of merit (FOM) to define a design space (i.e. combinations of IDTconductor thickness, pitch, and width) that provides XBARs withacceptable performance for use in filters. The FOM is calculated byintegrating the negative impact of spurious modes across a definedfrequency range. For each combination of IDT conductor thickness andpitch, the FOM is calculated for a range of IDT finger widths. Theminimum FOM value over the range of IDT finger widths is considered theminimized FOM for that conductor thickness/pitch combination. Thedefinition of the FOM and the frequency range depend on the requirementsof a particular filter. The frequency range typically includes thepassband of the filter and may include one or more stop bands. Spuriousmodes occurring between the resonance and anti-resonance frequencies ofeach hypothetical resonator may be accorded a heavier weight in the FOMthan spurious modes at frequencies below resonance or aboveanti-resonance. Hypothetical resonators having a minimized FOM below athreshold value were considered potentially “useable”, which is to sayprobably having sufficiently low spurious modes for use in a filter.Hypothetical resonators having a minimized cost function above thethreshold value were considered not useable.

FIG. 9 is a chart 900 showing combinations of IDT pitch p and IDT fingerthickness tm that may provide useable resonators. Both IDT pitch and IDTfinger thickness are normalized to the thickness ts of the piezoelectricplate. This chart is based on two-dimensional simulations of XBARs withlithium niobate diaphragms, aluminum conductors, and no dielectriclayers. XBARs with IDT pitch and thickness within unshaded regions 910,920, 930 are likely to have sufficiently low spurious effects for use infilters. XBARs with IDT pitch and thickness within unshaded regions 940,950, 960 are likely to have sufficiently low spurious effects for use infilters, but the IDT metal thickness is too low for use in high powerapplications. XBARs with IDT pitch and thickness within the interveningshaded regions have unacceptably high spurious modes for use in thetarget filter. With no dielectric layers, usable resonators exist forIDT finger thickness greater than or equal to 0.8 times thepiezoelectric plate thickness and less than or equal to 2.0 times thepiezoelectric plate thickness.

FIG. 10 is a chart 1000 showing combinations of IDT pitch and IDT fingerthickness that may provide useable resonators with a front-sidedielectric layer having a thickness tfd equal to 0.2 times thepiezoelectric plate thickness ts. The front-side dielectric layer may bea frequency setting dielectric layer deposited between the IDT fingersof a subset of resonators in a filter circuit, such as the shuntresonators 520A, 520B in the filter circuit of FIG. 5. In the chart1000, both IDT pitch and IDT finger thickness are normalized to thethickness of the piezoelectric plate. This chart is based ontwo-dimensional simulations of XBARs with lithium niobate diaphragms,aluminum conductors, and an SiO₂ front-side dielectric layer. XBARs withIDT pitch and thickness within unshaded regions 1010, 1020, 1030 arelikely to have sufficiently low spurious effects for use in filters.XBARs with IDT pitch and thickness within unshaded regions 1040 and 1050are likely to have sufficiently low spurious effects for use in filters,but the IDT metal thickness is too low for use in high powerapplications. XBARs with IDT pitch and thickness within the interveningshaded regions have unacceptably high spurious modes for use in thetarget filter. With a front-side dielectric layer having a thicknessequal to 0.2 times the piezoelectric plate thickness, usable resonatorsexist for IDT finger thickness greater than or equal to 1.1 times thepiezoelectric plate thickness and less than or equal to 2.0 times thepiezoelectric plate thickness. Usable resonators will exist for thisrange of IDT finger thickness for front-side dielectric layer thicknessless than or equal to 0.2 times the piezoelectric plate thickness.

FIG. 11 is a chart 1100 showing combinations of IDT pitch and IDT fingerthickness that may provide useable resonators with a front-sidedielectric layer, which may be a frequency setting dielectric layer,having a thickness equal to 0.3 times the piezoelectric plate thickness.Both IDT pitch and IDT finger thickness are normalized to the thicknessof the piezoelectric plate. This chart is based on two-dimensionalsimulations of XBARs with lithium niobate diaphragms, aluminumconductors, and an SiO₂ front-side dielectric layer. XBARs with IDTpitch and thickness within unshaded regions 1110, 1120, 1130 are likelyto have sufficiently low spurious effects for use in filters. XBARs withIDT pitch and thickness within the intervening shaded regions haveunacceptably high spurious modes for use in the target filter. UsableXBARs with thin IDT conductors do not exist. With a front-sidedielectric layer having a thickness equal to 0.3 times the piezoelectricplate thickness, usable resonators exist for IDT finger thicknessgreater than or equal to 1.15 times the piezoelectric plate thicknessand less than or equal to 1.8 times the piezoelectric plate thickness.Usable resonators will exist for this range of IDT finger thickness forfront-side dielectric layer thickness greater than 0.2 times thepiezoelectric plate thickness and less than or equal to 0.3 times thepiezoelectric plate thickness.

FIG. 12 is a chart 1200 showing combinations of IDT pitch and IDT fingerthickness that may provide useable resonators with a front-sidedielectric layer having a thickness equal to 0.35 times thepiezoelectric plate thickness. Both IDT pitch and IDT finger thicknessare normalized to the thickness of the piezoelectric plate. This chartis based on two-dimensional simulations of XBARs with lithium niobatediaphragms, aluminum conductors, and an SiO₂ front-side dielectriclayer. XBARs with IDT pitch and thickness within a small unshaded region1210 will have acceptably low spurious modes for use in filters. XBARswith IDT pitch and thickness within the surrounding shaded regions haveunacceptably high spurious modes for use in the target filter. UsableXBARs with thin IDT conductors do not exist. 0.35 times thepiezoelectric plate thickness is an upper limit for front sidedielectric thickness. No useful XBARs exist for materially thickerdielectric layers.

Description of Methods

FIG. 13 is a simplified flow chart showing a process 1300 for making anXBAR or a filter incorporating XBARs. The process 1300 starts at 1305with a substrate and a plate of piezoelectric material and ends at 1395with a completed XBAR or filter. The flow chart of FIG. 13 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 13.

The flow chart of FIG. 13 captures three variations of the process 1300for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 1310A, 1310B, or1310C. Only one of these steps is performed in each of the threevariations of the process 1300.

The piezoelectric plate may be, for example, rotated Y-cut lithiumniobate. The Euler angles of the piezoelectric plate are [0°, β, 0° ],where β is in the range from 0° to 60°. Preferably, β may be in therange from 26° to 34° to minimize coupling into shear horizontalacoustic modes. β may be about 30°. The substrate may preferably besilicon. The substrate may be some other material that allows formationof deep cavities by etching or other processing.

In one variation of the process 1300, one or more cavities are formed inthe substrate at 1310A, before the piezoelectric plate is bonded to thesubstrate at 1320. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1310A will not penetrate through the substrate.

At 1320, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1330 bydepositing and patterning one or more conductor layers on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1330 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1330 using alift-off process. Photoresist may be deposited over the piezoelectricplate and patterned to define the conductor pattern. The conductor layerand, optionally, one or more other layers may be deposited in sequenceover the surface of the piezoelectric plate. The photoresist may then beremoved, which removes the excess material, leaving the conductorpattern.

At 1340, a front-side dielectric layer may be formed by depositing oneor more layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 1300, one or more cavities areformed in the back side of the substrate at 1310B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 1300, a back-side dielectriclayer may be formed at 1350. In the case where the cavities are formedat 1310B as holes through the substrate, the back-side dielectric layermay be deposited through the cavities using a conventional depositiontechnique such as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 1300, one or more cavities in theform of recesses in the substrate may be formed at 1310C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device.

In all variations of the process 1300, the filter device is completed at1360. Actions that may occur at 1360 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at1360 is to tune the resonant frequencies of the resonators within thedevice by adding or removing metal or dielectric material from the frontside of the device. After the filter device is completed, the processends at 1395.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substrate;a lithium niobate plate having a front surface and a back surface facingthe substrate, a portion of the lithium niobate plate forming adiaphragm that spans a cavity in the substrate; and an interdigitaltransducer (IDT) on the front surface of the lithium niobate plate suchthat interleaved fingers of the IDT are on the diaphragm, wherein thelithium niobate plate and the IDT are configured such that a radiofrequency signal applied to the IDT excites a primary shear acousticmode in the diaphragm, Euler angles of the lithium niobate plate are[0°, β, 0° ], where β is greater than or equal to 0° and less than orequal to 60°, and a thickness of the interleaved fingers of the IDT isgreater than or equal to 0.8 times a thickness of the lithium niobateplate and less than or equal to 2.0 times the thickness of the lithiumniobate plate.
 2. The device of claim 1, wherein β is greater than orequal to 26° and less than or equal to 34°.
 3. The device of claim 1,wherein β is about 30°.
 4. The device of claim 1, further comprising adielectric layer between the interleaved fingers of the IDT.
 5. Thedevice of claim 4, wherein a thickness of the dielectric layer is lessthan or equal to 0.2 times the thickness of the lithium niobate plate,and the thickness of the interleaved fingers of the IDT is greater thanor equal to 1.1 times the thickness of the lithium niobate plate andless than or equal to 2.0 times the thickness of the lithium niobateplate.
 6. The device of claim 4, wherein a thickness of the dielectriclayer is greater than 0.2 times the thickness of the lithium niobateplate and less than or equal to 0.3 times the thickness of the lithiumniobate plate, and the thickness of the interleaved fingers of the IDTis greater than or equal to 1.15 times the thickness of the lithiumniobate plate and less than or equal to 1.8 times the thickness of thelithium niobate plate.
 7. The device of claim 4, wherein a thickness ofthe dielectric layer is less than or equal to 0.35 times the thicknessof the lithium niobate plate.
 8. The device of claim 1, wherein adirection of acoustic energy flow of the primary acoustic mode issubstantially orthogonal to the front and back surfaces of thediaphragm.
 9. The device of claim 1, wherein the back surface of thelithium niobate plate is attached to the substrate.
 10. The device ofclaim 9, wherein the back surface of the lithium niobate plate isattached to the substrate with an intermediate bonding layer.
 11. Afilter device, comprising: a substrate; a lithium niobate plate having afront surface and a back surface facing the substrate, portions of thelithium niobate plate forming one or more diaphragms spanning respectivecavities in the substrate; and a conductor pattern on the front surface,the conductor pattern including a plurality of interdigital transducers(IDTs) of a respective plurality of acoustic resonators, interleavedfingers of each of the plurality of IDTs on respective diaphragms of theone or more diaphragms, wherein the lithium niobate plate and all of theIDTs are configured such that respective radio frequency signals appliedto the IDTs excite respective primary shear acoustic modes in therespective diaphragms, Euler angles of the lithium niobate plate are[0°, β, 0° ], where β is greater than or equal to 0° and less than orequal to 60°, and the interleaved fingers of all of the IDTs have acommon thickness greater than or equal to 0.8 times a thickness of thelithium niobate plate and less than or equal to 2.0 times the thicknessof the lithium niobate plate.
 12. The filter device of claim 11, whereinβ is greater than or equal to 26° and less than or equal to 34°.
 13. Thefilter device of claim 11, wherein β is about 30°.
 14. The filter deviceof claim 11, further comprising a frequency setting dielectric layerformed between the interleaved fingers of a subset of the plurality ofIDTs.
 15. The filter device of claim 14, wherein a thickness of thefrequency setting dielectric layer is less than or equal to 0.2 timesthe thickness of the lithium niobate plate, and the common thickness ofthe interleaved fingers of the IDTs is greater than or equal to 1.1times the thickness of the lithium niobate plate and less than or equalto 2.0 times the thickness of the lithium niobate plate.
 16. The filterdevice of claim 14, wherein a thickness of the frequency settingdielectric layer is greater than 0.2 times the thickness of the lithiumniobate plate and less than or equal to 0.3 times the thickness of thelithium niobate plate, and the common thickness of the interleavedfingers of the IDTs is greater than or equal to 1.15 times the thicknessof the lithium niobate plate and less than or equal to 1.8 times thethickness of the lithium niobate plate.
 17. The filter device of claim14, wherein a thickness of the frequency setting dielectric layer isless than or equal to 0.35 times the thickness of the lithium niobateplate.
 18. The filter device of claim 14, wherein the plurality ofacoustic resonators includes one or more shunt resonators and one ormore series resonator connected in a ladder filter circuit, and thesubset of the plurality of IDTs is the one or more shunt resonators. 19.The filter device of claim 11, wherein respective directions of acousticenergy flow of all of the primary acoustic modes are substantiallyorthogonal to the front and back surfaces of the diaphragm.
 20. Thefilter device of claim 11 wherein interleaved fingers of each of theplurality of IDTs are disposed on a respective diaphragm spanning arespective cavity.
 21. The filter device of claim 11, wherein the backsurface of the lithium niobate plate is attached to the substrate. 22.The filter device of claim 21, wherein the back surface of the lithiumniobate plate is attached to the substrate with an intermediate bondinglayer.